Efficacy of the Specific Endothelin A Receptor Antagonist Zibotentan (zd4054) in Colorectal Cancer: A Preclinical Study

Endothelin 1 (ET-1) is overexpressed in cancer, contributing to disease progression. We previously showed that ET-1 stimulated proliferative, migratory, and contractile tumorigenic effects via the ETA receptor. Here, for the first time, we evaluate zibotentan, a specific ETA receptor antagonist, in the setting of colorectal cancer, in cellular models. Pharmacologic characteristics were further determined in patient tissues. Colorectal cancer lines (n = 4) and fibroblast strains (n = 6), isolated from uninvolved areas of colorectal cancer specimens, were exposed to ET-1 and/or ETA/B receptor antagonists. Proliferation (methylene blue), migration (scratch wounds), and contraction (gel lattices) were assessed. Receptor distribution and binding characteristics (Kd, Bmax) were determined using autoradiography on tissue sections and homogenates and cytospun cells, supported by immunohistochemistry. Proliferation was inhibited by ETA (zibotentan > BQ123; P < 0.05), migration by ETB > ETA, and contraction by combined ETA and ETB antagonism. Intense ET-1 stromal binding correlated with fibroblasts and endothelial cells. Colorectal cancer lines and fibroblasts revealed high density and affinity ET-1 binding (Bmax = 2.435 fmol/1 × 106 cells, Kd = 367.7 pmol/L; Bmax = 3.03 fmol/1 × 106 cells, Kd = 213.6 pmol/L). In cancer tissues, ETA receptor antagonists (zibotentan; BQ123) reduced ET-1 binding more effectively (IC50: 0.1–10 μmol/L) than ETB receptor antagonist BQ788 (∼IC50, 1 mmol/L). ET-1 stimulated cancer-contributory processes. Its localization to tumor stroma, with greatest binding/affinity to fibroblasts, implicates these cells in tumor progression. ETA receptor upregulation in cancer tissues and its role in tumorigenic processes show the receptor's importance in therapeutic targeting. Zibotentan, the most specific ETA receptor antagonist available, showed the greatest inhibition of ET-1 binding. With its known safety profile, we provide evidence for zibotentan's potential role as adjuvant therapy in colorectal cancer. Mol Cancer Ther; 12(8); 1556–67. ©2013 AACR.